1. Field of the Invention
The invention relates to a lighting arrangement for illuminating an area under mesopic conditions.
2. Description of the Related Art
Lighting for illumination of an area under mesopic conditions like utility lighting, e.g. street lighting, lighting used to illuminate parks, car parkings, gardens, and emergency lighting, as currently widely used, is designed to illuminate the relevant area in a way that provides an agreeable aura. Conventional light sources for utility lighting include incandescent, fluorescent and other discharge lamps.
Recently, alternative low-energy designs have been developed using LED source which are of considerably higher luminance, i.e. significantly more concentrated in terms of flux/mm2. This development has been focused on LEDs which generate white light. The white light is then formed by arranging interaction between light emitted by blue LEDs and a suitable phosphor.
Both conventional lamps and white LEDs based on blue LEDs combined with phosphors are not optimally designed with respect to the human eye at reduced light levels, i.e. under so-called mesopic light conditions.
The human eye has two types of photoreceptors. The first type of photoreceptors, called cones, is used for daytime vision. The second type of photoreceptors, called rods, is used for vision at reduced light levels together with the cones. The light level during daytime is generally such that cones suppress the rods. Hence, only the cones are used. However, the dominance of the cones diminishes if the light level is reduced. The rods become more dominant under the latter condition.
In international patent application WO2006/132533, a lighting arrangement is described which provides an improved visibility compared with conventional utility lighting. The lighting arrangement is designed to emit light in a first wavelength region and light in a second wavelength region. The lighting unit is further designed to generate light having a dominant wavelength from the first wavelength region in such a way that the eye sensitivity of the human eye is dominated by rods. Although, the lighting arrangement described in WO2006/132533, can improve vision at low intensity, further improvement is desired.